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Abstract

Background

The multiple pregnancy incidence is increasing worldwide. This increased
incidence is concerning to the health care system. This study aims to determine the frequency
of multiple pregnancy and identify factors that affect this frequency in Tehran, Iran.

Materials and Methods

This cross-sectional study included 5170 mothers in labor between July 6-21, 2015 from 103 hospitals with Obstetrics and Gynecology Wards. The
questionnaire used in this study consisted of five parts: demographic characteristics; information related to pregnancy; information related to the infant; information regarding
the multiple pregnancy; and information associated with infertility. We recruited 103
trained midwives to collect data related to the questionnaire from eligible participants
through an interview and medical records review. Frequencies and odds ratios (OR) for
the association between multiple pregnancy and the selected characteristics (maternal
age, economic status, history of multiple pregnancy in first-degree relatives, and reproductive history) were computed by multiple logistic regression. Stata software, version
13 (Stata Corp, College Station, TX, USA) was used for all statistical analyses.

Conclusion

The frequency of multiple pregnancy approximated results reported in previous studies in Iran. Based on the results, we observed significantly greater frequency of
multiple pregnancy in older women, those with a history of ART, and a history of multiple
pregnancy in the mother’s family compared to the other variables.

Introduction

The occurrence of twin and multiple pregnancies
has increased in developed countries (1)
and is associated with concern in the health care
system. Multiple pregnancy results in premature
delivery, underweight newborns, and increased
congenital anomalies. The worst outcome is
maternal and neonatal mortality (2). The existing
evidence shows a significantly lower
one-year survival in multiple infants compared
to singletons. The frequency of growth disorders,
as well as physical and mental disabilities is higher in multiple newborns, if the infants
survive (3, 4). A few studies conducted in Iran
have reported a frequency of twin pregnancy
from 1.5 to 8% (5-7). However, these studies
were frequently conducted in one or several
hospitals. Most were retrospective studies that
reviewed the records. Inconsistency in reporting
the frequency of multiple pregnancy could
be due to structural differences in the populations
studied and design effect and systematic
errors (selection or information bias), in addition
to changes in the frequency of the interested
outcome over time (8). Hence, it is necessary
to accurately identify the frequency of multiple
pregnancy and impacting factors which lead to
identification of high-risk groups and increased
care for these groups, and assists authorities and
policy makers in evidence-based decision making
to increase cost effectiveness of the interventions.
The aim of this study is to determine
the frequency of twin and multiple pregnancy
and to identify factors that affect the frequency
of this phenomenon in Tehran Province, one of
the main provinces in Iran.

Materials and Methods

We conducted a cross-sectional study in Tehran
Province, Iran which included the twenty-fifth
most populated city worldwide-the capital of Iran
(9). Participants comprised 5170 mothers in labor
between July 6 to 21, 2015 who referred to the
Obstetrics and Gynecology Wards of 103 hospitals.
These hospitals are affiliated with Tehran,
Beheshti, and Iran medical universities, which
oversee and mange 19 (Tehran), 43 (Beheshti),
and 41 (Iran) hospitals. We included all women
in this study regardless of the type of delivery
(natural or cesarean section) and the pregnancy
outcome (live birth, stillbirth, or spontaneous
abortion).

The Ethical Committee of Royan Institute approved
this study (EC/92/1097). All participants
received complete explanations about the study
aims and data confidentiality, which mentioned
their complete freedom to participate. Eligible individuals
were also assured that acceptance or refusal
to participate in the research had no influence
on their treatment procedures. Completion of the
questionnaire was considered as written informed
consent.

According to the 2% prevalence of multiple
pregnancy in the population (10), the effect size
of 0.006 and a design effect of approximately 2,
we estimated the required minimum sample size to
be approximately 4181 pregnant women (α=0.05).
The dependent variable studied was multiple pregnancy
(twin or higher). The questionnaire used in
the study included five parts: demographic characteristics
(13 items); information related to pregnancy
(26 items); information related to the infant
(15 items); information regarding the multiple
pregnancy (18 items); and information associated
with infertility (7 items).

Face and content validity

A total of 10 experts in gynecology, sexology,
and methodology assessed face and content validity
of the questionnaire. The validity index
for each question and total validity were calculated
(11). To equalize the experts’ perceptions of
content validity indices (relevancy, clarity, and
comprehensiveness of the tool), we sent the definitions
of these indices with the questionnaire.
Relevancy, clarity, and comprehensiveness were
defined as follows. Relevancy was the ability of
selected questions in order to reflect the content,
lucidity of the questions concerned their wording,
concept was clarity, and the instrument’s ability to
include all content domains or areas was considered
comprehensiveness. The experts were asked
to review clarity and relevancy of each item, and
comprehensiveness of the total questionnaire.
Scores were given as: 1 (inappropriate), 2 (somewhat
appropriate), 3 (appropriate), and 4 (quite
appropriate). Experts’ responses were gathered
within 1 to 3 weeks (12).

Data related to the questionnaire was collected
from eligible participants through interviews conducted
by 103 trained midwives. If pain was a barrier
to mothers’ responses to the questionnaire, data
were taken after childbirth at the time of admission
in the hospital, which took 24 hours. To ensure
valid and reliable data collection, the following actions
were taken. We conducted three training sessions
for midwives who collected the data. In these
sessions, the correct way to collect data, definition
of variables, and creation of a common perception
among midwives were considered. A pilot study
for operational feasibility and identification of implementation
problems and difficulties related to the questionnaire was conducted in five hospitals.
A visit to hospitals without previous coordination
for examining how to complete the questionnaires.

Statistical analysis

Categorical and continuous variables were
summarized as number (%) and mean (SD). The
frequency of multiple pregnancies was calculated
as the percentage of multiple pregnancies
by mother’s age, history of infertility, assisted reproductive
technique (ART), history of multiple
pregnancy in the mother’s family, history of multiple
pregnancy in the father’s family, the mother
born of multiple pregnancy, and the father born
of multiple pregnancy. Crude odds ratios (OR)
for the association between the selected characteristics
(maternal age, economic status, a history
of multiple pregnancy in first-degree relatives,
and reproductive history) and multiple pregnancy
were computed by univariate logistic regression.
In the analysis we considered hospitals as a cluster.
Multivariate logistic regression was used to
adjust OR simultaneously for the aforementioned
variables. Criteria for model building was based
on the Hosmer-Lemeshow method (13). Results
were presented as OR with 95 % confidence intervals
(CI) (14). The Hosmer-Lemeshow test
was used for goodness of fit of the model (15).
We used Stata version 13 (Stata, College Station,
TX, USA) for statistical analysis.

Results

In this study, the IRA (Inter Rater Agreement) relevancy
of the questions was 78.34% with a clarity
of 92.78%. The questionnaire had a total relevancy
of 86.23%, clarity of 87.48%, and comprehensiveness
of 82%. In this survey we examined 5170
eligible pregnant women. Among the examined
pregnancies, there were 5093 single cases and 77
multiple cases. Multiple pregnancy had a frequency
of 1.48% (95% CI: 1.19-1.85). Mothers had a
mean age of 29.23 years (95% CI: 29.08-29.38).
Mothers with single pregnancy had a significantly
lower mean (SD) age of 29.20 (5.46) compared to
30.98 (5.86) for mothers with multiple pregnancy
(P=0.004, Table 1,).

There were 237 (4.58%) cases treated with ART.
The frequency of multiple pregnancy was 1.15%
in women who did not receive ART (95% CI: 0.08-
1.49), while the frequency of multiple pregnancy
was 8.44% in women who received ART (95%
CI: 5.5-12.72). Using logistic regression analysis,
we estimated the OR for the association between
ART and multiple pregnancy to be approximately
7.88 (95% CI: 4.65-13.35, P<0.001). Hence, the
frequency of multiple pregnancy in women who
received ART was 7.88 times greater.

As seen in Table 2, a significant association
existed between variables such as mother’s age
(OR=1.04, 95% CI: 1.001-1.09, P=0.044), ART
(OR=6.11, 95% CI: 1.7-21.97, P=0.006), and history
of multiple pregnancy in the mother’s family
(OR=5.49, 95% CI: 3.55-9.93, P=0.001) with the
frequency of multiple pregnancy after controlling
for other variables in this table. No significant association
existed between the frequency of multiple
pregnancy and other variables. The goodness
of fit test was performed for the final version,
which showed a good fit of the model (Hosmer-
Lemeshow chi2=5.57, P=0.695).

Discussion

After remarkable reduction in multiple births
during the second half of the twentieth century,
most recently a steady increase exists in multiple
births and its adverse subsequent consequences
worldwide (16). Studies have shown that the majority
of this increase is due to the increased age at
pregnancy and the emergence of ART. In the United
States from 1972 to 1999, there were 6 times
more triplets and 12 times more multiples than the
past. If women who became pregnant at an older
age were considered in the calculation, the above
prevalence would increase approximately 50-60
times (17). Iran, like other developing countries,
has experienced major changes in the structure of
its population. The socio-economic development
and establishment of health care networks caused
major changes in indicators of population health
and epidemiology in Iran (18, 19). Demographic
information, mainly derived from the census10
years once in Iran, along with health indicators
confirmed the above mentioned. Fertility indicators
(birth and total fertility rate) showed that since
2000, Iran has experienced a downward trend and
the population growth rate has been close to one.
By taking into account the age composition of the
community, we have found that the population is
increasing in age, whereas the relative frequency
of marriage has decreased and the age of marriage
increased in both men and women (9). During the
last 10 years, no study has evaluated the frequency
of multiple pregnancy and its trend. With regard to
information obtained in a few studies, the results
have suggested a subtle increase in multiple births
in Iran. The highest frequency reported was 2%
estimated from the last study conducted in 2005 in
three large teaching hospitals in Tehran (10). In the
current study, multiple pregnancy had a frequency
of 1.48 (95% CI: 1.19-1.85), which approximated
the frequency reported in previous studies. The
rate has been affected by Genetics agents and ART.
Therefore, it differs in various regions of the world.
Bortolus et al. (20) carried out a systematic review
on the epidemiology of multiple births. The results
showed a higher frequency of multiple births
in African countries and the black race compared
with other countries. The lowest frequency was reported
from Japan and Southeast Asian countries.
Our results showed a moderate rate in Iran.

An international committee for monitoring ART
suggested that one embryo should be transferred
per cycle (21). Saraswat et al. (22) conducted a
systematic review in 2010. The results indicated
that infertility centers increased the number of
embryos transferred (sometimes up to 4 embryos)
according to domestic law and patient preference.
In the current study, the OR for an association between ART and multiple pregnancy was estimated
at 7.88 (95% CI: 4.65-13.35), which confirmed
findings of other studies (23). Another systematic
review on studies from 1950 to 2010 in the United
States revealed that 20% of twins, 40% of triplets,
and 71% of other types of multiple pregnancy
were caused by ovarian stimulation whereas 16%
of twins, 45% of triplets, and 30% of other types
were the result of IVF (16).

Martikainen et al. (24) conducted a study in four
centers in 2001. The results showed the clinical
pregnancy rate per transfer was 32.4% in the one
embryo transfer group and 47.1% in the two embryo
transfer group. The relative risk for twin birth
was 10.18. Mclernon et al. (25) reported a relative
risk for twin birth of approximately 24.4 (95% CI:
3.42-173.8), which indicated a very high risk for
twin pregnancy after the transfer of two or more
embryos. A review study conducted in 2002 by De
Sutter et al. (26) compared double embryo transfer
(DET) to single embryo transfer (SET) according
to the results of the 7407 cycles in 6 cohort studies.
Overall, the pregnancy success rate had no significant
difference between the two procedures (SET:
33.9%-DET: 35%) Twins were 1% in SET which
increased to 32.6% in DET. In the current study,
we have observed a direct association between
the mother’s age and multiple pregnancy. Age is
one of the risk factors for multiple pregnancy. The
trend for this type of pregnancy exactly depended
on the pattern of change in women's age at marriage
(27, 28).

Adashi et al. (29), in a research conducted in
the United States, found that 20% of the increase
in twin births was attributed to the reproductive
age of women. From the remaining 80%, ovulation
with fertility drugs comprised 40 and 40%
was attributed to IVF. In Denmark, an increase
in multiple births was seen exclusively in women
30 years of age and older; most of the pregnancies
were dizygotic. Blondel and Kaminski (30)
have reported a one-quarter to one-third increase
in twin pregnancies attributable to an increase in
reproductive age in women. The increase in reproductive
age is effective in twin birth of dizygotic,
but the rate of monozygotic pregnancies is
constant with changes in maternal age. This is due
to the increase in gonadotropin levels with increasing
age (31). Maximum follicle stimulation occurs
between the ages of 35-39 years, after which ovarian
function declines. Another finding of our study
was the strong association between multiple pregnancy
and a positive history of multiple births in
the mother’s family. This association was not seen
between multiple pregnancy and a positive history
of multiple birth in the father’s family. In a casecontrol
study in Italy the OR in women who had a
history of multiple pregnancy in their first-degree
relatives were 2.4 for dizygotes and 2.7 for monozygotes
(14). Baldwin (32), observed these findings
only in dizygotic twins. In a systematic review
by Bortolus et al. (20) on factors that affected
multiple births, an increase existed in the risk of
multiple pregnancy in those with a history of multiple
pregnancy in their first-degree relatives. The
findings were confirmed for dizygotic twins. The
mechanism of this association was explained in
1970 by Bulmer (33).

Our study was the first survey conducted with
the large sample sizes from both public and private
hospitals that had no selection bias (response
proportion: 100%). We attempted to hold the same
training session for interviewers (midwives) to
minimize information bias. A pilot study carried
out at the beginning of the study during over one
week detected operational problems, and examined
reliability and validity of the questionnaire.
Our study has several limitations. First, the crosssectional
nature of the study did not allow for conclusions
on causality due to the because of temporality
between the exposure and outcome. Second,
in this study we only assessed multiple pregnancy
without considering the type of multiple pregnancy
(i.e., monozygote or dizygote).

Conclusion

Based on the our study results, frequency of multiple
pregnancy in older women, women with history
of ART, and a history of multiple pregnancy in
the mother’s family had a significant relationship
with increased frequency of multiple pregnancy.
We observed no significant relationship between
the frequency of multiple pregnancy and other included
variables.

Acknowledgements

This study was the result of a project approved
and financially supported by Royan Institute. We
express our appreciation to the Treatment and Research
Departments of Tehran University of Medical Sciences, Beheshti University of Medical Sciences,
Iran University of Medical Sciences, and
Islamic Azad University. We also express our appreciation
to Dr. Fatemeh Sharif-Zadeh, Dr. Sadegh
Shariati-Nasab, and Dr. Mehrandokht Abedini,
as well as all of the midwives who contributed to
this study for data collection data. The authors declared
no conflict of interest.